Cleaner heads for vacuum cleaners are known to include a rotatable brush bar that operates to beat dirt and dust from textured floor coverings and the surfaces of furniture. Such a cleaner head performs particularly well at removing fibrous matter such as animal hair from textile floor or furniture coverings.
In such a cleaner head equipped with a brush bar, the brush bar may be driven either by an electric motor or by an air turbine which is powered by the flow of air through the cleaner head during use. Whether driven by either an electric motor or air turbine, it is common to transmit rotary power from the drive unit to the brush bar by a drive belt mechanism. An example of such a drive belt mechanism is shown in U.S. Pat. No. 4,976,003.
In such a drive belt arrangement, power from an input drive member is transmitted to an output drive member by a flexible toothed drive belt. The input and output drive members have parallel axes and the distance between the two members is fixed. Such an arrangement is known as a fixed centre drive, and is commonly used in production applications since they are relatively simple and cost-efficient.
In order for a fixed centre drive to operate reliably, the drive belt must be tensioned precisely. However, the accumulation of tolerances on the components of the drive mechanism can cause the drive belt, when assembled, to fall outside the relatively narrow range of acceptable tension values. This can be a problem since too high a belt tension can cause reduced component life in addition to causing assembly difficulties, whereas too low a belt tension can also prematurely wear the belt and the pulleys, as well as increasing the risk that the belt may slip. It is against this background that the invention has been devised.